1. Field of the Invention
The present invention relates to a metal powder, a conductive paste using the metal powder, a monolithic ceramic electronic component in which internal electrodes are formed using the conductive paste, and to a method for producing the metal powder.
2. Description of the Related Art
A monolithic ceramic electronic component, such as a monolithic ceramic capacitor, usually includes a ceramic laminate comprising dielectric ceramic layers, internal electrodes provided between the dielectric ceramic layers, and terminal electrodes electrically connected to the internal electrodes.
The ceramic laminate is formed by firing a green ceramic laminate in which a plurality of green ceramic sheets composed of a dielectric ceramic material are laminated. In order to form the internal electrodes, a conductive paste is printed on a plurality of the green ceramic sheets, and then the conductive paste is fired simultaneously with the green ceramic laminate. One of the ends of each internal electrode is exposed to one end face of the ceramic laminate. A conductive paste is applied to both end faces of the ceramic laminate so as to be connected to the exposed ends of the internal electrodes, followed by baking, to produce the terminal electrodes.
As described above, a conductive paste is used for forming internal electrodes of monolithic ceramic electronic components, in particular, monolithic ceramic capacitors. The conductive paste contains a metal powder which acts as a conductive constituent, and recently, noble metal powders such as silver powder or palladium powder, and base metal powders such as nickel powder or copper powder, have been used. As the size and thickness of monolithic ceramic electronic components decrease, a demand for production of fine metal powder, which is contained in conductive paste used for forming internal electrodes of the monolithic ceramic electronic components, is arising.
Conventionally, methods, such as a vapor phase process, for producing fine metal powder having a small particle size are used. However, since the cost of producing metal powder by the vapor phase process is high, there is a demand for producing metal powder having a small particle size by a liquid phase process.
For example, Japanese Examined Patent Application Publication No. 6-99143 discloses a method for producing metal powder by a liquid phase process in which a nickel salt solution is reduced in the liquid phase using a borohydride solution, such as a sodium borohydride solution, as the reducing agent, in order to obtain a metal powder having a small particle size.
However, since the boron in the reducing agent solution in the method disclosed in Japanese Examined Patent Application Publication No. 6-99143 is precipitated as an alloy or impurity in the metal powder, the purity of the resulting metal powder may be reduced. Consequently, the electrical properties of the metal powder are degraded, thereby making it unsuitable for use as the conductive constituent for forming internal electrodes in monolithic ceramic electronic components.
In order to inhibit the inclusion of impurities during the production of metal powder by a liquid phase process, for example, Japanese Unexamined Patent Application Publication No. 5-43921 discloses a method in which a solution containing basic nickel carbonate is reduced using hydrazine as the reducing agent. Since hydrazine is used as the reducing agent, impurities are not substantially precipitated in the metal powder during the reduction reaction. However, the minimum average particle size of the metal powder obtained by this method is 100 nm, which is too large for use as the conducting constituent to be contained in conductive paste for forming internal electrodes used for thinner monolithic ceramic electronic components.
The present invention has been achieved to overcome the problems described above. Objects of the present invention are to provide a method for producing a metal powder using a liquid phase process in which the resulting metal powder has an average particle size of about 100 nm or less and impurities are not substantially generated by a reducing agent, to provide a metal powder obtained by such a method, to provide a conductive paste using the metal powder, and to provide a monolithic ceramic electronic component using the conductive paste.
In one aspect of the present invention, a method for producing a metal powder includes the steps of preparing a reducing agent solution; preparing a mixed metallic salt solution in which a nickel salt and a copper salt are dissolved in a solvent; and mixing the reducing agent solution and the mixed metallic salt solution so that the copper salt is reduced to precipitate copper particle nuclei and then the nickel salt is reduced to precipitate nickel around the copper particle nuclei.
In accordance with the present invention, it is possible to produce a metal powder by a liquid phase process, which has an average particle size of about 100 nm or less and which is suitable for use in the formation of internal electrodes of monolithic ceramic electronic components.
Preferably, at least one of the nickel salt and the copper salt is at least one selected from the group consisting of chloride, sulfate and nitrate.
Preferably, the copper content is about 4.6xc3x9710xe2x88x927 to 5 percent by weight relative to 100% by weight of nickel in the mixed metallic salt solution.
Consequently, it is possible to reliably produce a metal powder which has an average particle size of about 100 nm or less and which is suitable for use in the formation of internal electrodes of monolithic ceramic electronic components.
Preferably, the reducing agent solution is prepared by dissolving a caustic alkali and at least one of hydrazine and hydrazine hydrate in a solvent, and more preferably, the caustic alkali is at least one material selected from the group consisting of potassium hydroxide, sodium hydroxide, calcium hydroxide, barium hydroxide and ammonia.
Consequently, it is possible to more reliably produce, by the liquid phase process, a metal powder which has an average particle size of 100 nm or less, in which impurities are not substantially generated by the reducing agent, and which is suitable for use in the formation of internal electrodes of monolithic ceramic electronic components.
Preferably, the solvent is a mixture of an alcohol and water. By using such a solvent, it is possible to control the particle size of the metal powder precipitated by changing the alcohol content in the solvent.
In another aspect of the present invention, a metal powder is produced by the method of the present invention described above. That is, nickel is formed around fine copper particles which act as nuclei in the metal powder.
Preferably, the copper content is about 4.54xc3x9710xe2x88x927 to 4.62 percent by weight relative to 100% by weight of nickel in the metal powder. Preferably, the metal powder has an average particle size of about 100 nm or less.
In another aspect of the present invention, a conductive paste contains the metal powder of the present invention and an organic vehicle. By using such a conductive paste, it is possible to form internal electrodes used for thinner monolithic ceramic electronic components.
In another aspect of the present invention, a monolithic ceramic electronic component includes a ceramic laminate including a plurality of dielectric ceramic layers and internal electrodes formed between the dielectric ceramic layers, in which the internal electrodes are formed using the conductive paste of the present invention.
In the monolithic ceramic electronic component of the present invention, internal electrodes which are thinner than conventional internal electrodes are provided, and it is possible to inhibit the delamination of the electrodes, decrease in coverage, occurrence of cracking and increase in the ESR.